func canvasContext(c jquery.JQuery) (x *canvas.Context) {
x = canvas.GetContext(c)
h := float64(c.Width())
w := float64(c.Height())
x.PixelsPerPoint = screenPixelsPerPoint
x.PointsPerPixel = 1 / screenPixelsPerPoint
x.Size = r2.XY(h*x.PointsPerPixel, w*x.PointsPerPixel)
// Always work in points not pixels.
x.Scale(r2.XY1(screenPixelsPerPoint))
return
}
